1. Field of the Invention
The present invention relates to a fuel supply system where pressurized fuel is supplied by a fuel pump from a fuel supply path to a fuel injector. In particular, the invention relates to a fuel supply system which is suitably applicable to a direct-injection engine where high-pressure fuel is directly supplied into a cylinder.
2. Background Art
In the field of internal combustion engines for automobiles, various techniques have recently been proposed as measures to improve the exhaust emission. For example, a technique disclosed in Japanese Patent Laid-open No. 2002-317669 (hereinafter referred to as “Patent Document 1”) is proposed as a solution to prevent fuel leakage from the fuel injector when the engine is stopped.
Generally, automotive engines are configured such that from a fuel tank, pressurized high-pressure fuel is supplied by a fuel pump to the fuel injector of each cylinder by way of a delivery pipe serving as a fuel supply path. A check valve is provided on the output side of the fuel pump so that the fuel pressure is kept high in the delivery pipe even if the fuel pump is stopped.
Due to the structure of the fuel injector, however, if the fuel injector continues to be subject to the high fuel pressure, it is possible that fuel may leaks from the fuel injector in terms of the configuration thereof. The fuel leaked while the engine is at rest may be emitted as unburnt fuel when the engine is started later. This may aggravate the exhaust emission. In particular, in the case of a direct-injection engine which directly supplies fuel into the cylinders, since the fuel leaked into the cylinders are directly emitted without being burnt if the cylinders are in the exhaust stage, the possibility of the fuel leak aggravating the exhaust emission is particularly high.
As such, disclosed in Patent Document 1 is a solution to prevent fuel from leaking from the fuel injector when the engine is stopped. According to this prior art technique, a fuel return path is added between the delivery pipe and the fuel tank and this return path is provided with an electromagnetic valve. When the engine is stopped, the electromagnetic valve is opened according to the fuel temperature estimated from the intake air temperature. This lowers the fuel pressure in the delivery pipe and therefore prevents the fuel injector from leaking fuel.
As described below, however, it is difficult to reliably prevent the fuel injector from leaking fuel merely by opening the electromagnetic valve according to the fuel temperature as mentioned above when the engine is stopped.
While the engine is running, the fuel in the delivery pipe not only receives heat from the engine but also is cooled by low temperature fuel supplied from the fuel pump. After the engine is stopped, the fuel temperature in the delivery pipe rises for a while since the fuel continues to receive residual heat from the engine although the fuel pump stops. It is therefore possible that after the fuel pressure is lowered by opening the electromagnetic valve, the fuel pressure may rise again due to the succeeding rise of the fuel temperature which causes the fuel in the delivery pipe to expand. The above-mentioned prior art techniques do not consider such a rise of the fuel temperature after the engine is stopped.
In addition, while above-mentioned prior art techniques open the electromagnetic valve for a predetermined period of time, this valve opening period, if not appropriate, causes the following problem.
While the engine is running, the fuel temperature is about 60° C. At this temperature, the saturation vapor pressure of the fuel is about 150 kpa. If the fuel pressure is lower than this saturation vapor pressure, the fuel boils to generate gas bubbles therein. While the engine is running, it is not likely to generate gas bubbles since the fuel pressure is higher than the saturation vapor pressure. If the electromagnetic valve is opened, however, the fuel pressure in the delivery pipe falls to the internal pressure of the fuel tank. Since the internal pressure of the tank is substantially equal to the atmospheric pressure (about 100 kpa) lower than the saturation vapor pressure of the fuel, the fuel in the delivery pipe boils to generate abundant gas bubbles therein if an excessively long valve opening period is set to the electromagnetic valve. Excessively abundant gas bubbles results in bad starting performance of the engine since they retard the rise of the fuel pressure when the engine is restarted.